Aren't the spells that too? This doesn't need specific mechanic, it is any magic such a pact grants.
And I repeat, warlock doesn't make sense metaphysically. If the pact imbues the character with power, such a person now possesses innate magical power and that's sorcerer's fluff. But if the power is constantly channelled instead, then that's a cleric. Warlock doesn't have metaphysics that separates it from other classes, thus shouldn't exist as separate class. Its mechanics are good though, and perfectly suit the sorcerer which has pointless mechanics. So rather than having two flawed classes, I'd rather combine them into one coherent and strong one.